(181c) Fabrication of High Density Carbon-Carbon Composites Using Polymer Infiltration and Pyrolysis

Carbon/carbon (C/C)-composites are composed of a carbon fiber reinforcement in a carbon matrix. C/C-composites are lightweight, have low thermal expansion, high tensile strength, and the ability to withstand up to 3000˚C before degradation making them useful materials in areas that demand structural integrity at extreme temperatures. C/C-composite can be found in the aerospace industry, automotive industry, defense industry, and miscellaneous other areas due to these properties. Their wide-spread use has caused their consumption rate to increase progressively since their first development in the 1960’s. In the fabrication process, the carbon matrix begins as a polymer precursor matrix. The initial carbon fiber reinforced polymer (CFRP) composite can be fabricated using hand layup, closed molding, filament winding, pultrusion, or other methods. Generation of the carbonaceous matrix is done through a process called pyrolysis. In pyrolysis, the polymer is heated in an inert environment to thermally onset the carbonization reaction. During the carbonization reaction, the polymer matrix experiences mass loss from the production of volatile gases while undergoing a volume change. When the force of the volatile gas diffusion and matrix volume change mechanism is stronger than the shear strength of the fiber-matrix bond and/or the tensile strength of the matrix, a porous network is generated through the material. This porous network reduces the integrity of the composite and diminishes the thermomechanical properties. Therefore, continual densification processes are used to increase the composites to a density of 1.6 g cm^-3, a common benchmark for these materials. These densification processes include multiple cycles of pressure assisted polymer infiltration and further pyrolysis of this newly infiltrated polymer. Through these processes, fabrication of C/C-composites can take days or even months depending on the size of the composite. The long lead time increases the cost to manufacture at a large scale. The inability to manufacture on a large scale makes C/C-composites effective systems but inapplicable for many commercial applications. Therefore, reducing the processing time needed to achieve high density is vital to overcoming these challenges. In this report, a 50% volume fraction (VF) 10-layer carbon fiber reinforced polymer (CFRP) composite was fabricated using resin transfer molding (RTM). The polymer matrix was a polybenzoxazine precursor with a char yield of 62.5 ± 1.6 %. at 1000 ℃ as reported by thermogravimetric analysis (TGA). The initial density of the CFRP was 1.49 g cm^-3 with 2 % porosity indicating good initial properties. Coupons were cut from the composite and carbonization was performed in a hinged tube furnace (HTF) at a ramp rate of 10 ℃ min^-1 to 1000 ℃ to yield the carbon-carbon interface. The C/C-composite after initial pyrolysis was 1.34 g cm^-3 with 18 % porosity. The coupons were infiltrated with the benzoxazine monomer precursor at approximately 150 psi for 15 minutes using a resin infiltration apparatus (RIA) built for this study. The monomer was cured and the C/C-composite coupons with newly infiltrated polymer was pyrolyzed further. Seven polymer infiltration and pyrolysis (PIP) densification cycles were performed on the composite coupons to achieve a final density of 1.56 g cm^-3 with 2 % porosity. Scanning electron microscopy (SEM) imaging of the surface confirmed densification of the porous network throughout the composite. Furthermore, permeability testing and microcomputed tomography (microCT) was conducted on the coupons after each PIP iteration to understand how the porous network was infiltrated and densified over the densification cycles. A relationship between the formation of microcracks from volume change of the matrix and densification efficiency was established that can be applied to future development of C/C-composites. The fabrication of a C/C-composite with densities approaching 1.6 g cm^-3 after seven densification cycles at low pressures is proposed to be a viable processing pathway for the fabrication of C/C-composites.